Using a free calling app on your smartphone, tablet, or computer won't use any of your plan's voice minutes, and you can use it over wifi even when you have bad reception. Some apps allow you to call phone numbers (including land lines) for free, while others can only be used to call users that have the same app installed.
A few important notes: A free calling app uses the device's data connection instead of voice minutes to make calls. Cellular data will be used if not on a wifi network. Make sure you have a large enough data plan or are on wifi before placing lots of audio or video calls. Also, these apps also do not support 911 calls.

The best free calling app for iPhone, iPad, & PC: Google Hangouts

Google Hangouts is the best free calling app due to its call quality, free calls to phone numbers in the US and Canada, and advanced features with a Google Voice number. The only downside is that Hangouts for Android doesn't support free calling.

Google Hangouts is a free messaging service that replaced Google Talk (aka gchat) in 2013. Hangouts supports instant messaging, video calls with up to 10 participants, and free calls to US numbers. Hangouts can be accessed on the mobile app for iPhone/iPad or on computers through the Gmail or Google+ websites. There is also a Chrome extension for computers that will alert you to incoming messages. Hangouts for Android supports messaging and video calls, but not free calling.

Hangouts also integrates with a Google Voice number if you have one (it's not a requirement though). Hangouts can be set to ring (through a push notification or with the fore-mentioned Chrome extension) for incoming calls to your Google Voice number. It will also display your Google Voice number on the caller ID when calling someone.

The call quality is good and the app is very stable. There are a few shortcomings though: free calling is not available on Android, incoming calls only display the number and not name (even if the call is from a contact in my Google contacts), recent calls disappear, there's no way to set favorite contacts, and it takes a few taps to get into the calling section of the app. Hopefully the app will get better with time as calling is more fully integrated into the Hangouts experience.

The best free calling app for Android: Talkatone or FreedomPop

There is no single free calling app for Android with the functionality of Hangouts and unlimited free calling. There are a few different options with limited free calling and texting though.

FreedomPop can be used to make free calls and texts to US phone numbers. It comes with 200 free minutes and 500 free texts per month, and you get a new number when signing up. It's a good looking app that works as advertised, but I find the voice quality a little lacking. FreedomPop is available for Android, iPhone, and iPad. FreedomPop also offers free cell plans.

Talkatone is very similar to FreedomPop. Texting and incoming calls are unlimited, but you only get 20 minutes of outgoing calls per month. An additional 60 minutes can be purchased for $1. Call quality is solid, but the app has a lot of options that can be confusing. Talkatone is available for Android, iPhone, iPad, and Kindle Fire.

The maggicApp by magicJack is a very simple app that you can make free unlimited domestic calls with. It's easy to use and all that's required to get up and running is an email address. There's no way to receive calls, but it's great if you need to make a phone call over wifi or cellular or run out of minutes every once in a while. However, this app wasn't very kind to my Android battery life. It's available for Android, iPhone, and iPad.

Free VoIP app-to-app calling

The following apps support making free calls to users that also have the app installed on their device.

Apple's iPhone, iPad, iPod Touch, and newer Mac's come with FaceTime preinstalled. FaceTime originated as a video-calling app only, but gained the ability to make audio calls on mobile devices later on. When initiating a call, just press the telephone icon instead of the camera icon and the call will go through as a voice call. FaceTime is easy to use and integrated within the native contacts app. However, you can only call other Apple users. It's a great way to make free calls, but it won't completely replace your phone plan.

The Facebook Messenger app for iPhone/iPad, Android, and Windows Phone includes free calling. When messaging someone tap on the little telephone icon to call them. You can also make video calls when on your computer, just tap the camera icon when messaging someone. Of course, you can only call other Facebook users.

Skype may be the best-known free calling app. Skype-to-Skype video and audio calls are free. You can also pay to call and text telephone numbers in the US and abroad from your computer or the Skype mobile app for iPhone, iPad, Android, and other devices.

Tango is similar to Skype, it offers free audio and video calls and messaging with other Tango users. Tango will scan your contacts list and automatically find other Tango users.

Wrap-Up: the best free calling app depends on your device and needs

The best free calling app will be different for iPhone and Android users. If you're like me and have an unlimited calling plan and most of your contacts have iPhones, FaceTime may be all you need. If I don't have great reception I use Hangouts on my laptop or iPhone. Try the different apps out and see what works for you, they're all free after all.